The present invention relates generally to the operation of control turn-off semiconductor devices and more particularly to a method and apparatus for determining, with respect to such a device, the control electrode to cathode junction voltage and the use of that determined voltage in the control of a signal utilized to render the device nonconductive.
Many power conversion and control systems presently in use employ semiconductor devices of the type generally referred to as control turn-off semiconductors. These semiconductor devices are of two general types, namely, power transistors and gate turn-off thyristors (GTOs). Although the GTO is more prevalent in today's applications, particularly in higher power applications, both device types have the common attributes of three electrodes which, although the commonly used terminology may vary, are basically an anode, a cathode and a control electrode, often referred to as a gate. These devices are rendered conductive (turned on) by a first type signal (normally positive) and rendered nonconductive (turned off) by a second type signal (usually negative). The magnitude of the signal required to turn a device off, however, and particularly in the case of the GTO, may be many times (e.g., 25 to 30) the magnitude of the signal required to turn the device on. For this reason, the greater problems are normally experienced with turning these devices off.
In many applications of the control turn-off semiconductor it is desirable to know the extant value of the control electrode to cathode junction voltage. A common purpose is to determine the conductive state of the semiconductor device for control purposes and for alarm or shutdown purposes, if the device is conducting at an improper time. An example of such a system is found in U.S. Pat. No. 4,641,231 "Apparatus and Method for Failure Testing of a Control Turn-Off Semiconductor" by Loren H. Walker and Georges R. Lezan, which patent is assigned to the assignee of the present invention and which patent is specifically incorporated hereinto by reference. That patent describes a typical power conversion scheme in which two semiconductors are connected in a series arrangement (commonly referred to as a leg) between the buses of a dc source. As illustrated in that patent, the commonly used three-phase converter has three legs each connected in mutual parallel between negative and positive dc buses. The semiconductors of the legs are rendered conductive in a predetermined order or sequence to control the ac electric power delivered from the bus to a load. If both semiconductors of any one leg become simultaneously conductive, it is apparent that a short circuit will exist between the two buses, which may result in damage to the load, the power source, and/or the semiconductors themselves.
The above cited patent employs the control electrode to cathode junction voltage in a scheme as just described. In that patent, a driver circuit (driver) is used to supply negative signals to the control electrode (gate of a GTO) to render the device nonconductive. It is assumed that the voltage at the output of the driver is the same as the control electrode to cathode junction voltage of the device itself. The value of this signal is directly compared to a fixed reference voltage to determine the conductive state of the device. That is, when the sensed voltage of this junction exceeds a value, that is an indication of a good device and a nonconducting or off condition. This indication, in the system described in that patent, allows the other semiconductor device or GTO of the leg to be turned on. In devices having relatively small current ratings, the direct sensing scheme as taught by the patent is entirely valid since the voltage drop due to lead inductance is relatively small with the smaller currents required to control the GTO. In fact, an inductance is often added to the driver circuit to limit the rate of current change (i.e., di/dt) and to protect the GTO. With higher rated devices, however, the magnitude of the current signal required to turn the device off (for example, 700 amperes) results in a voltage drop across the lead inductance (L di/dt) which may be so large as to obscure the true value of the control electrode to cathode voltage if the voltage at the output of the driver circuit is considered. While it is possible to employ a separate set of leads solely for purposes of voltage sensing, this is often not practical because of the particular physical configuration of the device and/or the structure of the heat sinks associated with the device.
A further problem with higher rated systems is that lead inductance severely limits the rate of change in the current (di/dt) for any given driver circuit voltage. While the use of a higher drive circuit voltage will provide sufficient di/dt for turn-off, if these higher voltages are allowed to remain across the control electrode to cathode junction once the semiconductor has been rendered nonconductive, the device may be destroyed because of excess reverse avalanche power.